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instructed, and these are:
·  0000 – malfunction
·  0030 – lost
·  0033 – parachute dropping
·  2000 – from non-SSR area
·  7000 – conspicuity code
·  7004 - aerobatics
·  7007 – open skies
In emergency, squawk:
·  7500 - Hijack
·  7600 – Communications failure
·  7700 - Emergency
You will be given details of other
traffic, on the clock system, such as
“fast mover at 6 o’clock”, based on
the track seen on the radar screen.
When changing squawks as
instructed, take care not to dial up
the emergency ones by mistake, and
do not switch the transponder to
standby during the change to avoid
it, as this will remove your display
from ATC’s screen.
A Mode A transponder is the regular
variety, which just displays the code
you select in the aircraft – you get
this by turning the switch to ON.
A Mode C is directly attached to an
encoding altimeter (or, more
precisely, an altitude digitiser, which
selects a different code to that
selected in the window), but only
Pressure Altitude information is sent
from the aircraft – the conversion to
local pressure, if required, is done
inside the ATC computer. ATC will
not be able to see changes when you
move the subscale. Mode C is
selected by switching to ALT.
If a transponder fails during flight in
a mandatory area, you may go to the
next planned destination, then
complete an itinerary or go to a
repair base, as permitted by ATC.
Electricity & Radio 135
It is possible to enter controlled
airspace without the required
equipment, but ATC must be asked
first. It is always subject to traffic.
When asked to squawk ident, your
return becomes brighter for a short
time, for positive identification.
Pilots report to the nearest 100', and
ATC confirm to the nearest 200'.
VDF
ATC can get a bearing for you to
steer (QDM) to get to their location
from your transmissions, when using
VHF Direction Finding equipment.
Being based on VHF, it is subject to
the usual limitations (see above), so
the higher you are, the better the
results you will get. You need to
transmit for a few seconds for a
bright line to spread from the centre
of a screen to the outside which is
marked with compass bearings.
·  QDM - magnetic bearing to
·  QDR – magnetic bearing from
·  QUJ – true bearing to
·  QTE – true bearing from
A VDF letdown exists where ATC
give you QDMs, and you work out
the headings to steer (a QGH is a
military letdown, where ATC give
you the headings to steer).
Older equipment will use a cathode
ray tube on which the line appears
(rather like a radar sweep) pointing
to where your transmission is
coming from. More modern digital
equipment uses a circle of LEDs at
10-degree intervals, which will show
the same information, with a digital
readout in the centre. The controller
can store the last transmission, in
case of being busy with something
else at the time. Radar can help by
giving distance information as well.
Accuracy comes in three classes, A,
B or C. Alpha is ± 2°, B is ±°5 and
C is ±10°. You will only be told the
classification if the bearing is B or C.
Radio Procedures
Like aircraft, pilots need radio
licences in order to use the airwaves.
This is normally a separate licence,
which needs you to pass an exam
before it is granted. Although your
use of the airwaves is limited (it's a
subset of the amateur radio
regulations), you still need to know
the phraseology so that other people
don't suffer.
Due to licence restrictions, aircraft
equipment is meant to have as few
controls as possible, including
displays, so some frequencies may
not be completely shown (122.075,
for example, comes up as 122.07).
The Squelch quietens down the
output when no signal is being
received, so you don't get
continuous earfuls of white noise. A
signal coming in cancels this and
activates the audio (a variable
squelch merely determines the signal
level when this occurs). The correct
procedure with the Squelch control,
therefore is to rotate the knob until
the hiss just stops, and leave it there,
although it is true to say that this will
　
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